I've been looking at some other spyshots of the M135i and I saw that the rear bumper isn't the same. Is it because that one is a 3-door version and the other is a 5-door? Or is there another reason? I like the one with the bumper in body color and not in black like its some cheap plastic.

The white one has just a standard F20 bumper. It's not wearing the M Sport kit, as seen on the blue 1 series. Any M135i will likely wear the M Sport aerodynamic package or some special aero kit of its own.